Studies by Mitchison, Benacerraf and Raff first suggested that physical interactions between Th and B-cells were essential in the development of humoral immune responses. Later studies documented that Th formed physical conjugates with class II major histocompatibility complex (MHC) compatible, antigen-presenting B-cells (Vitetta et al., (1987) Immunol. Rev. 99:193-239) and that it was the B-cells within these conjugates that responded to Th (Bartlett et al., (1989) J. Immunol. 143:1745-1754). With the discovery that Th-derived lymphokines exerted potent growth and differentiative effects on B-cells, it was proposed that soluble factor(s) released in proximity by activated Th mediated the activation of the interacting B-cell. However, none of the molecularly cloned lymphokines, alone or in combination, manifested the ability to induce B-cell cycle entry. Unlike soluble factors, plasma membrane fractions from activated Th induced B-cell cycle entry (Hodgkin et al., (1990) J. Immunol. 145:2025-2034; Noelle et al., (1991) J. Immunol. 146:1118-1124). Studies using purified plasma membrane fractions from activated Th suggested that a protein expressed on the membrane of activated Th was responsible for initiating humoral immunity (Noelle et al., (1991) J. Immunol. 146:1118-1124; Bartlett et al., (1990) J. Immunol. 145:3956-3962).
Purified plasma membranes from activated Th (PMAct) have been used to investigate the nature of this effector function (Hodgkin et al., (1990) J. Immunol. 145:2025-2034: Noelle et al., (1991) J. Immunol. 146:1118-1124). PMAct from activated Th, but not resting Th (PMrest) expressed an activity that induced B-cell cycle entry in an antigen-nonspecific, class II-unrestricted manner. In addition, it was shown that the activity expressed by PMAct required 4-6 hours of activation, de novo RNA synthesis and was protein in nature (Bartlett et al., (1990) J. Immunol. 145:3956-3962).